Various types of cells have been heretofore developed, and in every cell, a packaging material is a member that is absolutely necessary for sealing cell elements such as an electrode and an electrolyte. Metallic packaging materials have been often used heretofore as packagings for cells.
On the other hand, in recent years, cells have been required to be diversified in shape, and desired to be thinner and lighter as performance of electric cars, hybrid electric cars, personal computers, cameras and mobile phones has been enhanced. However, metallic packaging materials for cells that have been often used heretofore have the disadvantage that it is difficult to keep up with diversification of shapes, and there is a limit to weight reduction.
Thus, in recent years, a film-shaped laminate with a base material, a metal layer and a sealant layer laminated in this order has been proposed as a packaging material for a cell which is easily processed into diversified shapes and is capable of achieving thickness reduction and weight reduction. However, such a film-shaped packaging material is thinner than a metallic packaging material, and thus has the disadvantage that pinholes and cracks are easily generated during molding. When pinholes and cracks are generated in a packaging material for a cell, an electrolyte solution may permeate a metal layer to form a metal precipitate, resulting in occurrence of a short-circuit, and therefore it is absolutely necessary that a film-shaped packaging material for a cell be made to have such a characteristic that pinholes are hardly generated during molding, i.e. excellent moldability.
Various studies focusing on an adhesive layer for bonding a metal layer have been conducted heretofore in order to improve moldability of a film-shaped packaging material for a cell. For example, Patent Document 1 discloses a laminated packaging material including an inner layer composed of a resin film, a first adhesive layer, a metal layer, a second adhesive layer and an outer layer composed of a resin film, wherein at least one of the first adhesive layer and the second adhesive layer is formed from an adhesive composition containing a resin having an active hydrogen group on the side chain, a polyfunctional isocyanate and a polyfunctional amine compound, so that reliability of the packaging material for deeper molding is improved.
As exemplified by Patent Document 1, many studies have been conducted heretofore on techniques for improving moldability focusing on blended components of an adhesive layer for bonding a metal layer to other layers in a packaging material for a cell which is composed of a film-shaped laminate. However, there has hardly been reported on techniques for enhancing moldability by focusing on the physical properties of the metal layer.
Generally, a metal material having a low yield strength and a high tensile strength is known to have excellent processability such that the material is easily deformed, and hardly creased in deep drawing (see Non-Patent Document 1), and a metal material having a low yield strength has also been generally employed heretofore as a metal layer in a packaging material for a cell which includes a film-shaped laminate.